Conventionally, as a current measurement circuit in a constant voltage circuit, basically, a current measurement circuit 200 as illustrated in FIG. 4 has been used. In other words, an output of an operational amplifier 202 is connected for feedback to an inverting input terminal of the operational amplifier 202, thereby constructing a constant-voltage loop circuit and applying an output voltage, which is equal to a set voltage applied to an input terminal 212, on an output terminal 214. The feedback is applied to the output terminal 214 of the operational amplifier 202 so that the output voltage is a constant voltage equal to the set voltage applied to the input terminal 212, and is output to the output terminal 214. A current which flows through a device under test DUT (208) via a current detection resistor Rr (204), which is connected between the output of the operational amplifier 202 and the output terminal 214 and has a known resistance, can be measured by measuring a voltage between both ends of the current measurement resistor 204 at a current measurement terminal 216 via a differential amplifier 210.
In general, when an operational amplifier is driven to be saturated, it takes long time until the operational amplifier recovers. According to the method illustrated in FIG. 4, when a low current is to be measured, it is necessary for the current detection resistor 204 to have a large resistance. In this case, when the current flowing through the resistor only slightly increases, the voltage applied to the resistor increases largely.
Further, in this circuit, there exist parasitic capacitors due to considerable sizes of the current detection resistor 204 and peripheral circuits thereof. As a result of the influence thereof, when the set voltage largely changes, a large transient current flows, whereby the operational amplifier 202 in the constant-voltage loop tends to be saturated, and consequently, the measurement cannot be performed in this case. Moreover, when a low current is to be measured, the current detection resistor having a large resistance is inserted, and hence the load current cannot be large. Accordingly, there poses a problem that it takes a long period of time until the load current reaches and stabilizes at a steady-state value, and the operational amplifier recovers from the saturation, and returns to the state for correct measurement.
FIG. 1 of Japanese Patent Application Laid-open No. Sho 63-82377 discloses a circuit for solving this problem. For the sake of description, the related figure is herein illustrated as FIG. 5. In FIG. 5, reference numeral 10 denotes a load for which a current is measured, and a constant-voltage loop circuit 12 applies a load voltage V1 for measuring the current. In the configuration of the constant-voltage loop circuit 12, a DC voltage V2, which is specified by a digital signal 18 from a digital/analog converter 16 so as to be converted to an analog signal, is applied to an inverting input terminal of an operational amplifier 14 via a resistor 20, and the load voltage V1 is fed back via a voltage follower constructed by an operational amplifier 22, and a resistor 24. A non-inverting input terminal of the operational amplifier 14 is grounded. An output of the operational amplifier 14 is connected to an input of a current booster (buffer amplifier) 26, and an output thereof is connected to a load connection terminal 28.
The gain of this constant-voltage loop circuit 12 is determined by values of the resistors 20 and 24, and the load voltage V1 is maintained to a constant value proportional to the DC voltage V2.
In a current path serial to the load 10 in the constant-voltage loop circuit 12, in other words, between the output of the current booster 26 and the load connection terminal 28, a current detection resistor R1 is inserted, and current detection resistors R2 to Rn for changing the range are provided so as to be selectively inserted in parallel by respective reed relays RY1 to RYm. On this occasion, the resistor R1 is a current detection resistor for the minimum range, and a reed relay RYn is provided in parallel with the resistors R1 to Rn so as to short-circuit the current measurement resistors R1 to Rn.
A relay control circuit 30 controls, according to a digital control signal 32, the reed relays RY1 to RYn.
An operational amplifier 34 is provided for detecting a voltage drop of the current detection resistors R1 to Rn, the output voltage of the current booster 26 is applied to a non-inverting input thereof, the load voltage V1 is applied to an inverting input thereof via a voltage follower constructed by an operational amplifier 36, and an output voltage V3 of the operational amplifier 34 is a voltage proportional to the load current.
When the load 10 is capacitive, and a predetermined voltage is applied to the load 10, the reed relay control circuit 30 connects resistors R2 to Rn according to a measurement range to the resistor R1 in parallel, and at the same time, the reed relay for short circuit RYn is closed.
As a result, even for a small current measurement range, a transient current of the load 10 is not limited, and the load current is stabilized to a steady-state value in a short time. Moreover, the load current is not limited, and hence the operational amplifier 14 and the current booster 26 in the constant-voltage loop circuit 12 will not be saturated.
In this way, when the period of time required for the load current reaching the steady-state value has elapsed from the application of voltage, the relay control circuit 30 opens the reed relay for short circuit RYn, whereby a voltage proportional to the load current is measured as the output voltage V3 of the operational amplifier 34.
However, an operation speed of the reed relays is low in the constant-voltage loop circuit 12 illustrated in FIG. 5, and thus, it is impossible to efficiently avoid in advance the saturation of the operational amplifier, and to efficiently reduce the recovery time from the saturation of the operational amplifier. This is because it takes at least 250 microseconds for each operation of opening or closing the reed relays. Further, it takes a time to convert the output voltage V3 of the operational amplifier 34 into the digital control signal 32 for the drive control of the relay control circuit 30.